Telephony



1941- 'r. s. SKILLMAN z-rrm. 2,251,958

TELEPHONY Filed Jan. 8, 1938 Patented Aug. 12, 1941 ITELEP HONY v Thomas Samuel Skillman, Jan Willem Baurdoux,

' Y and Mathias van de Beck, Eindlioven, Netherlands, assignors, by memo assignments. to Hartford National Bank and Trust Company,

\ l Hartford, Court, as trustee Application January 8, 1938, Serial No.

Our invention relates to telephone receiving apparatus, and more particularly to a combined receiving, amplifying. and signalling device.

In low-frequency telephony, as well as in carrier-wave telephony over wires, it is well known to transmit in addition to the speech currents,

signals such as ringing -currents and selection impulses. In carrier-wave telephony such signals are modulated on the carrier-wave used to transmit the speech current, and after being demodulated and amplified, are supplied to a socalled signal receiver, and in low-frequency line telephony the signal receiver is connected directly to the line. As the signal receiver must respond only to signal currents and not to speech currents of the same frequency, it. is the usual practice to utilize the diflerence in amplitude or in duration between the signal currents and the speech currents. However, signal receivers operating on this principle are complicated and expensive.

The object of the present invention is to overcome the above difliculties and to provide a very simple and inexpensive receiving apparatus.

In accordance with the invention, which utilizes the fact that an amplifier is always present in both low-frequency and carrier-wave telephony receiving apparatus for amplifying speech currents, I provide in the input circuit of the amplifier an oscillatory circuit which is tuned to the signal frequency, and is connected to a rectifier and a resistance. The signal currents produce across this resistance a voltage drop which controls the grid voltage, and thus the anode current of the amplifying tube of the amplifi'er, to thereby energize a relay included in the anode circuit and controlling the signal current circuit.

In order that the invention may be clearly understood and readily carried into effect, I shall describe the same in more detail with reference to the accompanying drawing, in which:

Figure 1 is a schematic view of a portion of a telephone receiving apparatus embodying the invention; and I Fig. 2 is a schematic diagram of an apparatus according to another embodiment of the invention.

The receiver-amplifier illustrated in Fig. 1 comprises an input transformer I having a pri mary winding M, which in the case of lowrfrequency telephony is connected directly to a transmission line, and in the case of carrier-wave telephony is connected through a demodulator and the required filters. As such connections are Germany January 29,1937 2 Claims. (cl. 179-84) well known in the art further illustration or description of the same is believed unnecessary.

Transformer I has a secondary winding l5 connected in the grid circuit of an amplifying tube 3, and for this purpose has one end connected directly to the grid H and its other end connected through a rectifier 6 and a battery l2 to the cathode l3.

Tube 3 has a plate I6 connected to one end of the primary winding ll of an output transformer 2, the other end of winding ll being connected through a condenser lit to a cathode l3. A control relay 9 having a normallyclosed contact adapted to control a signal current circuit l0 has one end of its coil connected to a point between winding H and condenser ll,

and its other end connected to the positive side of a plate current supply (not shown).

In accordance with the invention, I connect across secondary winding IS a series-parallel circuit arrangement comprising a resistance 4 and having two parallel branches, one branch being an oscillatory circuit 5 comprising a. condenser l9 and an impedancelll, and the other comprising a condenser 8 and a resistance I. The incoming signal currents produce an alternating voltage across circuit 5, which voltage is rectified by rectifier 6, so that a direct voltage is set up across resistance 1 and condenser 8. This direct voltage changes the bias of grid II, to thereby decrease the plate current of the tube and cause contact 2| to close circuit Hi.

When a signal frequency of more than 2,000 cycles per second is used, and when circuit, 5 is tuned to this frequency, there is little damping effect, and this circuit hasa sharp resonance curve with the result that the speech currents will not produce a suflicientvoltage drop across resistance 1 to cause contact 2| to close. 'This appears to be due to the much smaller amplitude and duration of the speech oscillations in comparison to these values for the speech current.

I prefer to use for impedance 20, a coil having an iron core (as shown) for the following reahigh and therefore the hysteresis and eddy-current losses and the damping will increase accordingly. Thus the circuit 5 will have a less sharp resonance curve for the signal currents.

' r 2,251,958 high amplitude, the magnetic induction will be The above is of course very, important because a i is makes it necessary to employ-special measures for maintaining either the natural frequency of circuit 5 or the signal frequency, constant, while,

at the same time insuring that relay 9 will be correctly actuated. Thus if the resonance curve were too broad, the relay 9 might respond to the speech currents.

- The apparatus shownjn Figure 2 is similar to that of Figure 1, and operates. in the same man'- ner. However, in Fig. 2 the oscillatory'circuit frequency, a rectifier, and a resistance.

51 which is tuned to the signal frequency is con nected in parallel with winding ll, softhat series resonance instead of parallel resonance is used.

For this purpose condenser I9 and impedance 2!] are connected in series across winding 14, and rectifier 6 is connected between a tap 22 on impedance 20, and a point 23 between windings l5 and resistance 1 and condenser 8;

While we have described our invention in connection with specific examples and applications, we do not wish to be limited thereto, but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What we claim is:

1. A telephony receiving apparatus comprising a low frequency amplifier including a grid-controlled amplifier tube, said amplifier having a grid circuit to which are supplied the received speech frequency band and signal currents of a .2. A telephonyv receiving apparatus comprising a .-low-'frequency amplifier including a grid-controlled amplifier tube, said amplifier having a grid circuit tdwhich are supplied the received speech fr eque-ncy bands and signal currents of a 'frequency within the speech-frequency band and of amplitudes which are large relative to those of the speech currents, and an output circuit adapted to be connected to utilizationv means for the amplifiedspeecn current; controlling means in said output circuit tocontrol a signal circuit, said controlling means being controlled by the anode current of said tube, and means in said grid circuit and associated with the grid of said tube to change the grid voltage when signal currents are being supplied, said latter means comprising a rectifier, a resistance, and an oscillatory circuit 1 tuned to the signal frequency and includinga coil having an iron core.

THOMAS SAMUEL SKILLMAN. JAN WILLEM BAURDOUX. MATI-HAS VAN as BEEK. 

